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Understanding Stress in Geology: Folds, Faults, and Earthquake Dynamics

This text explores the relationship between different types of geological stress—compressive, tensile, and shear—and how they lead to various fold types and faults in the Earth's crust. It highlights the accumulation of stress over time from tectonic forces, resulting in earthquakes when that stress surpasses rock strength. Additionally, it discusses the mechanics of fault movements, the propagation of rupture during seismic events, and examples of significant faults like the San Andreas. The information is valuable for understanding the geological processes that shape our planet.

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Understanding Stress in Geology: Folds, Faults, and Earthquake Dynamics

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  1. Part 8: Fold Types

  2. Compressive Stress Shear Stress Tensional Stress Orientation of stress leads to different folds

  3. Ductile Strain

  4. Earthquakes Stanford, 1906

  5. Compressive Stress Tensional Stress Shear Stress

  6. Earthquakes occur on faults Active Fault

  7. Faults Faults are fractures with movement

  8. Faults Faults are fractures with movement

  9. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Time Earthquakes

  10. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Earthquakes are the result of stress that builds up over time. Local rock strength Stress Time Earthquakes

  11. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. Local rock strength Stress Time Earthquakes

  12. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. When the stress exceeds the strength of the rocks… Local rock strength Stress Time Earthquakes

  13. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. When the stress exceeds the strength of the rocks… …the fault slips, causing an earthquake. Local rock strength Stress Time Earthquakes

  14. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. When the stress exceeds the strength of the rocks… …the fault slips, causing an earthquake. Local rock strength The process repeats again and again. Stress Time Earthquakes

  15. 0 Seconds Rupture expands circularly on fault plane, sending out seismic waves in all directions. Focus Fault cracks at surface 5 Seconds Rupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state. Fault crack extends 10 Seconds The rupture front progresses down the fault plane, reducing the stress. 20 Seconds Rupture has progressed along the entire length of the fault. The earthquake stops.

  16. World seismicity from 1976 to 2002 ASIA EUROPE NORTH AMERICA PACIFIC OCEAN ATLANTIC OCEAN AFRICA SOUTH AMERICA INDIAN OCEAN AUSTRALIA ≤ 50 km deep (shallow focus) 50–300 km deep ANTARCTIC OCEAN ≥ 300 km deep (shallow focus)

  17. Plate Boundaries and Faults • Strike-slip faults along transform boundaries • Reverse faults along convergent boundaries • Normal faults along divergent boundaries

  18. Strike-Slip Faults: Transform Boundary • San Andreas Fault • Several magnitude 7 earthquakes recently • usually occur in clusters • arrows show motion/yr

  19. Faults not always parallel to boundary Results in uplift of mountains

  20. Denali fault, Alaska Mt. McKinley

  21. Offset streams along the San Andreas Fault in the Carrizo Plain, CA

  22. Southern California fault traces San Andreas fault San Gabriel Mountains North American Plate Pacific Plate Los Angeles Motion of Pacific Plate relative to motion of North American Plate

  23. Southern California fault traces Here, the San Andreas fault is parallel to plate motion, and the faulting is right-lateral strike slip. San Andreas fault San Gabriel Mountains North American Plate Pacific Plate Los Angeles Motion of Pacific Plate relative to motion of North American Plate

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